Hope you didn't get it from a store recently but don't answer that question...

If it was kept in the box and away from light it is likely OK.  Lines in general last a few seasons but that depends on how often you use it and how aften you clean it.
I am on my 2nd season with a never cleaned Sharkskin line and it likely needs some TLC but I want to beat it up (Read lazy). I will replace it this month.

Back in the day we were all complaining about how our lines only lasted a season but the Mfgrs seen to have made better coatings these days.

If it glides nicely through the guides and floats well its fine, but I would probably use a good line cleaner to replace any lost slickness. 
I am sure however that I do not own a 6 year old line.... huh, I do have a wulff TT6wt on my Leonard that has to be that old +.  Still working well but doesn't get out much.
Poor mistreated lonely rod.  Always in the dark though.  Maybe I'll take it out for a spin.
Thanks for reminding me.
Enjoy the "New" line.
PS - any line that looks dirty and still is dirty after cleaning take a good look because it is likely not dirt but tiny cracks in the surface of the line.  These tiny cracks usually run perpendicular  to the length of the line.  Water will penetrate and sink the tip as the core gets waterlogged.  Time to replace has long expired!
cheers

If you notice cracks soon just send it to the company they will replace it for you. I did that with a SA Line that did not even last a season that was only used for steelheading, they told to do a letter and send the line back, two weeks my line arrived in the mail.

Line cracking can also come just from the water you fish, hitting rocks as well but I have used different lines for the same length of time and never had that happen except to the SA line, might of gotten a bad batch.